Fuzes employed in explosive projectiles use many different design criteria in order to preclude prematurely arming or detonating the explosive due to spurious or other non-firing events. For example, two sensed independent physical environments, which are directly associated with the launch cycle, must have changed before the explosive projectile is allowed to become armed. For the purposes of this application, the term "armed" will be given its usual meaning which is well known to those of ordinary skill in the art. Briefly, however, the process of "arming" means that one or more certain predetermined events have occurred which allows for the enabling of the fuze function.
As those skilled in the art will appreciate, the two changed independent physical environments are commonly referred to as a "first environment" and a "second environment." Such environments generally relate to the sensing of objective physical conditions which cannot be duplicated except through firing the explosive projectile. Thus, the sensed environments must represent significant changes from the ambient conditions under which the projectile is manufactured, handled, transported, stored, and loaded for firing in order to ensure compliance with the proper safety considerations.
Oftentimes, the first environment which is sensed represents an actual firing event--such as the setback inertial force on the fuze caused by the forward acceleration of the projectile within the bore. Since the first environment relates to an event which occurs upon firing, it is desireable that the second environment represents the actual exit of the projectile from the bore in order to ensure in-bore safety (i.e., it is desireable to insure that there is no possibility of arming the fuze with subsequent initiation of the explosive train within the bore).
In the past, second environments which were sensed included set-forward acceleration (i.e., based on the negative acceleration of the projectile due to air friction/drag), or a specified level of spin (i.e., based on the spin imparted on the projectile due to the rifling of the bore to promote in flight projectile stability). However, these methods have suffered from drawbacks such as requiring moving parts and other unreliable mechanical sensors. In addition, the actual sensed parameters of such selected environments may suffer from varying magnitudes over all the conditions encountered.
Further, the second environment mechanical sensing devices discussed above were not capable of reliably initiating the timing of fuze functions beginning with the exit of the projectile from the bore. Finally, with the advent of electronics' based fuzes, such systems did not provide a simple and reliable electronic or other non-mechanical sensor which could be utilized to further the evolution/maturation to the complete elimination of all moving parts in the fuze.
Many tank gun projectiles employ a sabot as an integral part of the round design. The sabot is typically comprised of three "petals" which fit together around the actual projectile to fill the space between the projectile and the gun tube. The sabot provides structural integrity and sealing functions during the firing of the round, and is thereafter discarded upon muzzle exit to minimize flight weight and drag. While the manner in which sabots are discarded is known in the art, a brief example will next be provided.
The petals of the sabot are pinned together (at the trailing edge) about the round. A band, such as nylon, is wrapped around the sabot petals to hold the petals against the housing. Typically, there are no physical connections between the sabot and the housing. The petals of the sabot form an air-scoop at or near the leading edge of the projectile. Upon firing the projectile, the band breaks due to the environment within the gun tube. As the projectile exits the tube, the air scoop entrains air such that the petals are lifted up and away from the housing. This action shears the pins at the trailing edge of the petals, and so the petals fall away from the round.
As just described, one of the events which occurs upon the projectile leaving the bore is that the sabot releases. Therefore, detecting the release of the sabot would enable a meaningful solution to second environment safety considerations and initiating further timing functions (i.e., since the sabot release indicates that the projectile is out of the bore, it also inherently provides a meaningful initial time reference for safe separation and other further functions of the projectile's fuze).
Although sensing the sabot release is desireable, there is a dilemma presented in detecting the presence or absence of the sabot either by mechanical means or by remotely located electrical means, such as a switch. More specifically, all such detection indications must be routed inside the fuze well and into the fuze itself from points external to the projectile. However, wires and other mechanisms leading from the fuze to the outside of the projectile body present problems in manufacture, assembly, test, and firing survival of the fuze and projectile, as well as posing in-bore safety problems if the round is not well sealed.
Therefore, there arises a need for a second environment sensor apparatus and method which is capable of reliably sensing a second environment condition change to ensure safety by precluding arming of the fuze throughout the mission life of the projectile up to and including the in-bore time of the launch cycle, preferably without use of mechanical means or remotely located electrical means. Further, there arises a need for a second environment sensor apparatus and method which is capable of accurately generating a signal corresponding to the sensed condition and to preferably utilize the sensed condition signal to initiate the timing of fuze functions. The present invention directly addresses and overcomes the shortcomings of the prior art.